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solgato avatar image
solgato asked

BMV-712 Capacity Settings

Alright, now I’m a bit confused and maybe this post will help others down the line so my question is about the capacity setting when setting up a BMV-712. The unit has a default of 200ah and the manual states this is at the 20hr rating.

Not all battery manufactures provide the specs with the same terminology or scale of rating, and on top of that there is often a Reserve Capacity (RC) which is often described as being @ a certain Amp load measure of scale, and sometimes that is provided as opposed to a Capacity.

My question is, what specification does the 712 need when setting it up?

At first I thought the I could use the RC value which my manufacturer rates in minutes @ a 25A load. But digging around I was finally able to find a Capacity spec, however the scale of measure is not provided. The manual states that the 712 wants a figure based on a 20 hour rating which makes me think not all manufactures use the same scale of measurement.

I imagine others will run across this issue and so I thought I would post this for clarity since run of the mill store bought batteries don’t always offer all the specs, yet some of the terminology is similar which can make things even more confusing.

For my application, which is using my bank to power brushless electric trolling motors, Reserve Capacity is the important spec I use when shopping for a battery because the motors draw heavy loads for a long period of time and they get used heavily in a single outing, whereas some may be more interested in small loads and how long that load can be drawn from battery before discharging it to a state that is unhealthy.


Thx.

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Mark avatar image
Mark answered ·

The BMV will always interpret the capacity that you have entered as a 20h discharge capacity - there is no way to change this.

This does not mean that it simply uses this capacity for all calculations without any automatic compensation/adjustment to factor for the actual discharge rates that may be much faster or slower.

The Peukert Exponent (that is also entered into the BMV) is the factor in a more complex formula that the BMV uses to calculate the instentaneous 'corrected' battery capacity, based on the real discharge rate at any point in time (since real current draw is not typically constant), so that the State Of Charge % reading is as accurate as reasonably possible.

If required, the Peukert Exponent can be adjusted to make the capacity 'compensation' more or less agressive, but it's normally set to 1.25 for lead acid based batteries. A factor of 1 disables the compensation.

So in summary, you should aim to find out / determine the battery capacity that coinsides with a constant 20 hour discharge rate and use that if you want the BMV to calculate SOC% as accurately as possible.

If only a single capacity is provided for a battery it's normally the 20h rating. If only a 10h capacity is provided, that's normally fairly close, so OK to use in that case, or you could easily factor it up to an approximate 20h capacity. If your using a car battery for example, then most are not designed for deep cycling and it's common for no capacity rating to be provided, just CCA.

Once the BMV is initially setup, you typically still need to review & fine tune some BMV parameters over the coming days / usage cycles, once you have an idea about how it perform's / behaves - ie. how well the BMV tracks the true SOC & if a 100% SOC sync is achieved when required (not too early before the battery is really full & that it does actually occur).

I hope this helps & answers your questions.

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solgato avatar image solgato commented ·

Thanks Mark.

So then I should assume that the 88 Amp Hour spec I was able to hunt down for my Deep Cycle 27 marine batteries is measured at a 20 hour discharge rate, and that I and others should ignore the Reserve Capacity (RC) which is often provided in lieu of Capacity for deep cycles marine type batteries when setting up the BMV-712?

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Mark avatar image Mark ♦♦ solgato commented ·

Yes that would be a very reasonable assumption. But if you want to confirm you can normally contact the manufacturer directly or via the distributor.

Reserve capacity assumes 25A of current draw, regardless of battery size to provide a time / hour rating. Although useful it can not be directly translated to an Ah capacity rating at 20h discharge rate (unless by luck/chance the reserve capacity time is 20h...).

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